Zirconium is used in the form of alloys as a cladding material for fuel rods in a nuclear reactor, and furthermore is a valuable metal used for e.g. a white pigment and a piezoelectric element as oxidized zirconium. In the meantime, scandium with the lowest atomic number among the rare earth elements is used as e.g. a material for metal halide lamps, an additional element in alloys and, in recent years, an additional element in catalytic ceramics.
Furthermore, the use of ScSZ (Sc2O3-doped ZrO2/Scandia-stabilized zirconia) as an electrolyte for solid oxide fuel cells called SOFC has been recently considered as a use common to both zirconia and scandium.
As described above, both zirconium and scandium are valuable elements and the efficient recovery and purification of them have been attempted. In the future, it is also considered that used SOFCs be recycled to separate, recover and reuse scandium and zirconium.
As a method used for such separation, for example, it is considered that an electrolyte, ScSZ, for the above used SOFCs is dissolved by adding a strong acid, impurities are separated using a means such as neutralization-precipitation, and furthermore scandium and zirconium are separated by a means such as solvent extraction.
Patent Document 1, for example, describes a method for separating iron and/or zirconium from actinide and/or lanthanide existing in an aqueous acid solution by a propanediamide. This method uses a 5-substituted propanediamide having the structure represented by the following chemical formula:
(wherein, R1, R2 and R3, which may be the same or different, are an alkyl group which optionally has one or two oxygen atoms in the chain.)
Patent Document 1 above, however, does not describe the behavior of scandium, which belongs to Group 3 elements together with the lanthanoids but has chemical properties slightly different from each other, and there is not knowledge to separate zirconium and scandium.
For the separation of zirconium and scandium, Patent Document 2 provides a technique for separating Sc3+ from an aqueous solution containing Sc3+ and metal ions other than Sc3+ using a versatile reagent, and particularly discloses carrying out the step of adding an organic solvent and a first chelating agent to form a complex with Sc3+ in the organic solvent to an aqueous solution containing Sc3+ and metal ions other than Sc3+, the step of forming a mixed solution by mixing the aqueous solution and the organic solvent to form a complex of Sc3+ and the first chelating agent, and the step of separating the mixed solution into the organic phase and the aqueous phase.
When using the method described in Patent Document 2, however, as shown in the text and FIGS. 1 to 7 in Patent Document 2, for example, scandium is extracted instead of zirconium, or the extraction behaviors of iron and aluminum partially have the same tendency as the extraction behaviors of zirconium, and thus it cannot be said that this is sharp separation, and there is a problem in that it is difficult to obtain high-purity zirconium.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H05-70856
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2013-57115